1. Field of the Invention
The present invention relates to an alkali secondary battery and, more particularly, to an alkali secondary battery whose positive electrode containing nickel hydroxide as an active material is improved.
2. Description of the Related Art
A sintered positive electrode has been conventionally used as a positive electrode to be incorporated into an alkali secondary battery. This sintered positive electrode is manufactured by sintering nickel grains on a two-dimensional substrate, such as punched metal or a nickel mesh, impregnating holes ten-odd .mu.m in diameter of the resultant porous plate with an aqueous nickel salt solution, and converting the impregnated nickel salt into nickel hydroxide by an alkali treatment.
The manufacture of the above sintered positive electrode, however, requires complicated active material impregnating operations such as the nickel salt impregnating step and the alkali treatment step. Additionally, these operations must be repeated four to ten times in order to impregnate a predetermined amount of an active material. This results in an increased manufacturing cost. Furthermore, if the porosity of the nickel grain sintered body obtained by the sintering as discussed above exceeds 80%, it is difficult to maintain the mechanical strength of the sintered body. Therefore, increasing the filling amount of the active material has its limit.
For these reasons, it has been attempted to manufacture a positive electrode by preparing a paste by adding a conductor, a binder, and water to nickel hydroxide grains and mixing the resultant material, and filling this paste into a metal porous body having a three-dimensional structure, such as a sponge-like metal porous body or a metal fiber mat with a mean porosity of 95% or more and a mean pore size of a few ten to a few hundred micrometers. The positive electrode manufactured by this method is called a nonsintered positive electrode (or a paste positive electrode) in comparison with the sintered positive electrode. The metal porous body of this paste positive electrode has a porosity and a mean pore size larger than those of the sintered positive electrode. This advantageously facilitates filling of the active material and can increase the filling amount of the material.
In the paste positive electrode, however, pores of the metal porous body, such as a sponge-like metal porous body, into which the paste is filled, are larger than those of sintered nickel. This increases the distance between the active material and a collector, resulting in a low conductivity. In addition, since concentration of the current density is caused as the absolute amount of the active material increases, the positive electrode swells especially in overcharge. Consequently, the utilization ratio of the positive electrode decreases, and this decreases, e.g., the charge/discharge efficiency.
For the above reasons, in order to take advantage of a large capacity of the paste positive electrode, it is necessary to (a) increase the conductivity, (b) suppress the swelling ratio of the positive electrode, and (c) increase the charge/discharge efficiency.
Increasing the conductivity is very important in decreasing the mean charge/discharge polarization potential of the positive electrode during charge/discharge of an alkali secondary battery.
The conductivity is improved by addition of metal cobalt or a cobalt compound, such as cobalt oxide or cobalt hydroxide.
Of the other two problems, (b) suppressing the swelling ratio of the electrode is a problem especially in overcharge, and (c) increasing the charge-discharge efficiency is a problem particularly at high temperatures. In solving the problem (b), it is important to minimize production of low-density .gamma.-nickel oxyhydroxide (.gamma.-NiOOH) as one higher order oxide during overcharge. In solving the problem (c), it is important to increase the oxygen overvoltage of the positive electrode at high temperatures, so that charge electrical energy is not partially consumed in generating oxygen gas.
To solve the above problems (b) and (c), addition of a transition metal or a transition metal compound is adopted in the formation of the sintered positive electrode. Well-known examples of the transition metal element to be added are cadmium (Cd) and cobalt (Co). Known examples of the method of adding this transition metal element are a method (coprecipitation addition method) of solidly dissolving the transition metal element, together with nickel atoms, into nickel hydroxide grains, and a method (mixing addition method) by which transition metal grains or grains of a transition metal compound (primarily oxide or hydroxide) are mixed together with nickel hydroxide grains during kneading in the paste preparation step. From an environmental point of view, however, a general consciousness for banning hazardous components of batteries has become acute recently. For example, the regulations for even a very slight amount of cadmium contained in the positive electrode of a nickel-hydrogen secondary battery have become stricter. A demand therefore has arisen for cadmium-free batteries.
As methods meeting this requirement, Jpn. Pat. Appln. KOKAI Publication No. 2-30061 has disclosed a method of coprecipitating and adding zinc or a zinc compound in place of cadmium, and Jpn. Pat. Appln. KOKAI Publication No. 3-77273 has disclosed a mixing addition method of this type. A positive electrode containing nickel hydroxide grains manufactured by the former zinc coprecipitation addition method is effective in decreasing the swelling ratio. However, a positive electrode containing nickel hydroxide grains manufactured by the latter zinc mixing addition method has no distinct effect of decreasing the swelling ratio. Additionally, when used at high temperatures, the low-rate charge efficiency of the positive electrode containing nickel hydroxide grains manufactured by the zinc coprecipitation addition method or the zinc mixing addition method is lower by about 15 to 20% than that of the positive electrode containing nickel hydroxide grains added with an equal quantity of cadmium. Consequently, although a cadmium-free positive electrode can be obtained, (c) increasing the charge/discharge efficiency noted previously has not been basically achieved yet when the positive electrode is incorporated into a battery.
Jpn. Pat. Appln. KOKAI Publication No. 5-21064, on the other hand, has disclosed an alkali storage battery which includes a positive electrode containing nickel hydroxide grains consisting of a mixture of spherical or almost spherical grains and aspherical grains containing at least one of Cd, Ca, Zn, Mg, Fe, Co, and Mn, and in which the capacity density and the cycle life of the positive electrode are improved by discouraging swelling of the positive electrode.